Many modern high speed Local Area networks (LAN) systems have adopted burst Orthogonal Frequency Division Multiplexing (OFDM) as their physical layer. Examples of communications standards based on burst OFDM include 802.11a and hyperLan II standards. It is well know in the art that while OFDM is excellent for handling severe multipath, its performance is extremely sensitive to frequency offsets. Any mismatch in the receiver local oscillator and the transmitted signal can cause a loss of orthogonality between the carriers, this can lead to severe Inter-Channel Interference (ICI). Such mismatch is sometimes called a carrier frequency error since the oscillator is not synchronized with the carrier frequency of the transmitted signal.
Because of the potential loss of orthogonality between the carriers, carrier recovery is one of the most critical functions to perform when processing OFDM signals correctly.
FIG. 1 illustrates an exemplary signal structure 100 for the 802.11a wireless standard. FIG. 1 outlines the various OFDM symbols that make up a typical 802.11a signal. The signal structure 100 is separated into three fields, a preamble 134, a signal field 136 and a data field 138. The preamble field 134 comprises the first 16 μs 102 of the 802.11a signal structure 100. Burst OFDM signals use a preamble field 134 of the illustrated type because the signals are bursty in nature. The preamble facilitates channel estimation. In contrast, continuous OFDM signals need not use preambles since their relatively long durations provide time for channel estimation to be performed without the aid of a preamble. The preamble field 134 has two sections, the first section 114 is 8 μs 104 in duration and includes 10 short, repeating units of data (t1 through t10 each 16 samples long at 20 Mhz sampling rate, and all being identical in value). The second section 116 is also 8 μs 106 in duration and includes two OFDM symbols, T1 and T2, packed together with a guard interval, GI2, twice the normal length. The guard interval GI2 protects the payload information from distortions due to multipath, ICI, etc.
The data from the time interval 124, which includes the first half of the first section 114 of the preamble 134, gives a receiver time to recognize that a valid burst OFDM signal is being received. The data from the time interval 126, which includes the second half of the first section 114 of the preamble 134 is normally used for coarse frequency offset estimation and timing synchronization. The data included in the second section 116 of the preamble, corresponding to time interval 106, is used for channel estimation.
The preamble 134 is followed by a signal field 136, which is an OFDM symbol that is always binary phase shift key (BPSK) modulated. The signal field 136 is 8 μs 108 long. The data in the signal time interval 108 is used for determining the rate length.
Finally, the signal field 136 is followed by a data field 138 which includes a collection of OFDM symbols 120, 122 (maximum 1365) that can be modulated using a plurality of different modulation scheme, e.g., BPSK, QPSK, 16QAM or 64QAM. These OFDM symbols 120, 122 are each 8 μs 110, 112 in length. They include a first field, GI, which is a guard interval and a second field, Data 1, Data 2, which includes data. The duration of the first field, GI, is 0.8 μs, and the duration of the second field, Data 1, Data 2, is 3.2 μs each. The data received in the time interval 132, corresponding to data field 138, is used to determine the service being used and includes the data that is trying to be sent. At the beginning of each signal field 136, system parameters, e.g., carrier offset, FFT frame time, sampling rate offset, gain control, etc., are known for proper processing of the 802.11a signal. In other words these system parameters are derived from the information sent in the preamble 134 and therefore available for use in processing signal field 136.
Burst and continuous OFDM signals need excellent carrier recovery systems to be implemented in their respective receivers because of their short signal duration and for other reasons mentioned earlier. In the art, most OFDM carrier recovery systems use what is known as the cyclic prefix correlation approach.
In burst OFDM, the OFDM symbols that carry data are processed immediately after the preamble. Given this fact and the bursty nature of burst OFDM signals, a distributed, time-averaged approach over the entire received signal to determine the true carrier offset is usually not an option. This problem is particularly troublesome under low SNR conditions. It would be desirable if a method were available where carrier frequency correction could be based, at least in part, on the data transmitted immediately following a preamble.
Therefore there is a need for a robust carrier recovery system, e.g., that intelligently processes more sections of the signal to achieve more accurate carrier offset determinations in the relatively short amount of time available.